Review Ho & Gao
from standards (surrogate matrix spiked with analyte), authentic tissue blank spiked with analyte, to incurred tissue samples [3,5,11–12,17]. Specificity is evaluated by examining the chromato-
grams of blank samples. Both the processed blank sur- rogate and blank tissue samples are injected to ensure that there is no interference or false positive peak at the retention time of the analyte [4,18,21]. Some reports evaluated the specificity using multiple lots of blank tissue and over a range of tissue weights [5,6]. Interference components in surrogate and in tissue
can be different. When the validated method for the determination of arachidonic acid and its metabolites in rat brain was applied to incurred sample analysis, an interference peak was found for one of the analytes [11]. Szerkus et al. reported the validated method for the quantitation of cyclosporine A in ocular rabbit tis- sue could not be used to analyze some of the samples due to the strong interference by the incurred sample matrix [12].
Linearity, precision & accuracy Linearity is evaluated by preparing standards in sur- rogate matrix at 6–8 concentration levels with 2–3 replicates at each level [7,31]. Often, multiple batches are prepared to evaluate the intra- and interday linearity. The acceptance criteria reported by all authors is that back-calculated concentrations must be within 15% except for the LLOQ sample for which 20% is accept- able [3,27]. In some cases, linearity is evaluated con- currently with the determination of surrogate matrix validity as previously discussed. Precision and accuracy should be evaluated for sur-
rogate matrix and for authentic tissue. The impor- tance of evaluating both matrices is demonstrated by Teunissen et al., who observed significant bias for QC in tissue but not in surrogate matrix and made method adjustment based on the observation [27]. Many peo- ple do evaluate precision and accuracy in both matri- ces [7,11,21–22,27]. However, some only evaluated for sur- rogate [3,12–14,16,27,30] or tissue [5]. In certain cases, the lack of precision and accuracy evaluation for tissue is because the suitability of surrogate matrix has been established [14,17], but not always [12]. The precision and accuracy determination are typi-
cally done by preparing QC samples at 2–4 levels with 3–6 replicates at each concentration [3–5,10–11,16–17,27]. The exact concentration and number of levels do not have to be the same for surrogate and tissue QC [11]. Often, both intra- and interday precision and accu- racy are assessed [12,17,28]. In addition, some people also evaluate sample dilution [7,22]. Like linearity, the acceptance criteria reported by all authors are the same, which is within 15%. Uniquely for tissue analysis,
2426 Bioanalysis (2015) 7(18)
some evaluated precision and accuracy using tissue of different mass and different lots of tissue. Zhang et al. assessed two different tissue masses (low and high) to bracket the expected actual tissue mass range for unknown samples [22]. They performed precision and accuracy evaluation at three concentrations for each tissue mass. Niklowitz evaluated the precision of three lots of tissue. Triplicates were prepared for each lot and the run-to-run variations over 5 consecutive days were evaluated [23].
Extractability & recovery Extractability of analyte from surrogate and from tissue can be different. For example, Edpuganti et al. found that binding of analyte to the phospholipid in tissue affected the measured concentration, which was not applicable to the ethanol surrogate [13]. For this reason, recovery is one of the important parameters for evalu- ating a surrogate matrix assay. Technically, the recov- ery should be evaluated for both surrogate and tissue. In practice, only recovery from the tissue is evaluated in most cases. As mentioned by Xue et al. the recovery is most often done on homogenate and rarely done in solid tissue [38]. Generally, recovery is performed at 2–3 concentrations with 3–6 replicates at each concentra- tion. Most people do not have acceptance criteria for minimal recovery but rather are seeking reproducible and consistent recovery between surrogate and tissue. One group has reported a minimal requirement of at least 70% recovery [51]. Matuszewski et al. differentiated the recovery
(%RE) and process efficiency (%PE) [52]. The %RE is also referred to as extraction efficiency or extraction recovery, is the response ratio of a compound spiked in the sample or matrix before extraction to that of spiked after extraction. The %PE is also known as total recovery, or analytical recovery, is the response ratio of a compound spiked in the sample or matrix before extraction to that of neat solution. The %RE is the true recovery because it takes into account the effect of matrix. However, in the case that the surrogate matrix happens to be the neat solution, the %RE and %PE of analyte from surrogate matrix will be the same. In the literature, there are roughly equal numbers of people reporting the recovery as %RE [13,28] as those who report %PE [16,18]. There are two schools of
thoughts for recovery
(%RE) determination when recovery is determined using incurred tissue samples or control matrix con- taining endogenous analyte [4,28,52]. Both are illus- trated in Figure 2. One method requires determining the responses of pre- and postextraction spiked sam- ples (pre- and postsamples). Recovery is calculated by directly comparing the responses of the two samples
future science group
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154